Aqueous two-package type clear coating composition and process for the formation of multilayer finish coating film

Abstract

The present invention provides an aqueous two-package type clear coating composition comprising: (A) an aqueous dispersion of a hydroxyl group- and acid group-containing acrylic resin having a hydroxyl value of 30 to 200 mg KOH/g, an acid value of 5 to 50 mg KOH/g, a weight average molecular weight of 3,000 to 30,000, and a glass transition temperature of −30 to +40° C. obtained by radical polymerization of 10 to 50 mass % of a secondary hydroxyl group-containing monomer (a) and 50 to 90 mass % of at least one other unsaturated monomer (b); and (B) a polyisocyanate curing agent obtained by mixing (c) a polyisocyanate compound and (d) a compound containing an anionic functional group, a polyoxyethylene group, and a hydrocarbon group, and a process for forming a multilayer topcoat film using the same.

The invention claimed is: 
     
       1. An aqueous two-package type clear coating composition comprising:
 (A) an aqueous dispersion of a hydroxyl group- and acid group-containing acrylic resin having a hydroxyl value of 30 to 200 mg KOH/g, an acid value of 5 to 50 mg KOH/g, a weight average molecular weight of 5,000 to 20,000, and a glass transition temperature of −30 to +40° C. obtained by radical polymerization of 10 to 50 mass % of a secondary hydroxyl group-containing monomer (a) and 50 to 90 mass % of other unsaturated monomer (b), 
 wherein the acrylic resin is obtained by solution polymerization, and the solution polymerization reaction is performed by a two stage polymerization method; and 
 (B) a polyisocyanate curing agent obtained by mixing (c) a polyisocyanate compound and (d) a compound containing an anionic functional group, a polyoxyethylene group, and a hydrocarbon group. 
 
     
     
       2. The coating composition according to  claim 1 , wherein the curing agent (B) is obtained by mixing the compound (c) and the compound (d), the compound (d) being 0.5 to 25 parts by mass per 100 parts by mass of the compound (c). 
     
     
       3. The coating composition according to  claim 1 , wherein the monomer (a) is at least one member selected from the group consisting of 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and adducts of (meth)acrylic acids and epoxy group-containing compounds. 
     
     
       4. The coating composition according to  claim 3 , wherein the monomer (a) is 2-hydroxypropyl (meth)acrylate. 
     
     
       5. The coating composition according to  claim 1 , wherein the aqueous dispersion (A) comprises an acrylic resin obtained from monomer (a) and monomer (b),
 wherein the monomer (b) comprises 5 to 30 mass % of at least one (meth)acrylate having a branched C 8-18  alkyl group. 
 
     
     
       6. The coating composition according to  claim 1 , which further comprises a hindered phenolic-based antioxidant (C). 
     
     
       7. The coating composition according to  claim 1 , which further comprises an ultraviolet absorber (D). 
     
     
       8. The coating composition according to  claim 1 , which further comprises an active hydrogen-containing light stabilizer (E). 
     
     
       9. The coating composition according to  claim 1 , wherein the lowest viscosity as measured at a temperature of 30 to 150° C. at a frequency of 0.1 Hz is 30 Pa·s or less. 
     
     
       10. The coating composition according to  claim 1 , wherein the melt viscosity of the aqueous dispersion (A) as measured at a solids content of at least 96 mass % at a shear rate of 564 s −1  at 140° C. is 1 to 12 Pa·s. 
     
     
       11. The coating composition according to  claim 1 , wherein the equivalent ratio (NCO/OH) of isocyanate groups of the curing agent (B) to hydroxyl groups of the acrylic resin in the aqueous dispersion (A) is 0.5 to 2.0. 
     
     
       12. The coating composition according to  claim 1 , wherein the solids content is 35 to 65 mass %. 
     
     
       13. A process for forming a multilayer topcoat film comprising forming on a substrate at least one base coat layer and at least one clear coat layer in that order,
 wherein the aqueous two-package type clear coating composition according to  claim 1  is applied as the uppermost top clear coat composition. 
 
     
     
       14. A process for forming a multilayer topcoat film, comprising applying as a base coating composition a thermosetting aqueous coating composition to a substrate, applying the aqueous two-package type clear coating composition according to  claim 1  as a clear coating composition to the uncured base coat surface, and then curing the resulting two coating layers simultaneously.